Evolution of the Earth's Atmosphere
| Stage | Time [yrs] of first occurrence | Duration [yrs] | Description |
| Original |
- 4.6 ´ 109 yrs |
Earth formation time |
Earth formed
(see also 2
and
3) in the inner solar system where
gases and ices were scarce. Refractory materials (mostly minerals) dominated. Hence the nascent earth had a sparse atmosphere, probably just a tenuous layer of H, He, and H-rich gases such as H2, NH3 and CH4. This atmo- sphere was soon lost as it escaped into space because of the high surface temperature of the young earth and the lightness of these molecules. |
| Outgassing & Accretion (Ahrens doesn't even mention accretion, but some experts believe it may have been as important as outgassing in contributing to the atmosphere.) |
Since formation, but acquired mostly in
first |
Volcanic eruptions have added much CO2,
H2O and N2 and lesser amounts of other gases. Cometary impacts have added ices of CO2, H2O, NH3 and CH4 which immediately vaporized to become part of the atmosphere. Photodissociation of these last three molecules as well as volcanically produced H2O increased atmospheric H, O, N and C. Most of the H escaped while chemically active O combined with other atoms to form a little O2, but more H2O and more CO2. As time passed most of the H2O condensed out of the atmosphere to form oceans. Much of the CO2 dissolved in the ocean and eventually was deposited in sedimentary carbonate rocks such as limestone or metamorphic rocks such as marble (metamorphosed limestone). This left N2 as the principal atmospheric molecule. |
|
| Photosynthesis |
-3 to -2 ´ 109 yrs |
From onset until present |
The first atmospheric O2 may
have formed as described in the box above, but, with the development of plant life, photosynthesis began to add O2 to the atmosphere at a much greater rate, building to essentially the current O2 concentration several ´ 108 years ago. |
